An image sensor is a semiconductor device that converts an optical image into an electrical signal, and is largely classified into a Charge Coupled Device (CCD) and a Complementary Metal Oxide Silicon (CMOS) image sensor.
The CMOS image sensor is a device that employs a switching method for sequentially detecting an output of each unit pixel using a Metal-Oxide-Semiconductor (MOS) transistor. In the CMOS image sensor, MOS transistors corresponding to unit pixels are formed on a semiconductor substrate through a CMOS technique using a control circuit and a signal processing circuit as a peripheral circuit.
A semiconductor region of the CMOS image sensor is typically divided into a photodiode region and a transistor region. The photodiode region converts light into electrons. The transistor region constitutes a circuit for driving an output of a unit pixel of the image sensor.
For example, a method of forming a photodiode using a donor substrate will be described. First, a single crystal silicon substrate is used as a donor substrate, and then hydrogen ions may be implanted in the donor substrate with a predetermined depth or depth profile.
Second, the donor substrate is attached on a semiconductor substrate having a metal interconnection, and then is divided into two portions by a smart-cut method. At this point, the donor substrate is cut into two parts based on a layer where hydrogen ions are implanted. The attached portion (from the donor substrate) on the semiconductor substrate with the metal interconnection is used as a photodiode layer.
Third, a contact hole for connecting the metal interconnections is formed in the photodiode layer and a dielectric layer overlying the metal interconnection layer.
The photodiode layer is a layer where hydrogen ions are implanted, as mentioned above, and its ion distribution may vary according to the region or depth thereof. That is, a separation portion or region of the photodiode layer has the largest number of defects due to ion implantation in the photodiode layer. As it approaches toward a region adjacent to the metal interconnection layer, defects due to ion implantation decrease.
Due to a defect distribution difference, when an etching process is performed to form a contact hole, an etch rate of the photodiode layer may vary according to the depth. Therefore, since a tapered hole having a broad top and a narrow bottom is formed, when a subsequent tungsten deposition process is performed, a void may be formed, and tungsten may be partially removed during the subsequent etching and/or cleaning process. Therefore, the contact hole may not be completely filled.
This may deteriorate the operational reliability of the image sensor because it affects the flow of current to the metal interconnect.